The present invention relates to a staged process and apparatus for producing glass or the like, and more specifically, to improvements in the refining stage of such a process or apparatus. Although specifically applicable to production of vitreous glass products such as flat glass, fiber glass, container glass, or sodium silicate glass, the invention is also applicable to similar products that may not be considered "glass" under strict definitions. It should be understood that the term "glass" is used herein in the broader sense to include glass-like products. On the other hand, because of the higher standards for optical quality of flat glass, the improvements in refining achieved by the present invention are particularly significant to the production of flat glass.
In U.S. Pat. No. 4,381,934 to Kunkle et al. there is disclosed a process for performing the initial step of the melting process, rendering pulverulent batch materials to a liquefied, partially melted state. This process requires that the melting process be completed by a subsequent process stage for most glass products. Refining of the liquefied material would be a typical task of the subsequent process stage. In the aforesaid patent, it is disclosed that the refining may be carried out by feeding the liquefied material to a conventional tank-type melting furnace. In order to optimize the economies of construction and operation of such a staged melting and refining operation, it is desirable to carry out the refining in as efficient a manner as possible, thereby minimizing the size of the refining apparatus and the energy consumed therein.
In the melting of glass, substantial quantities of gas are produced as a result of decomposition of batch materials. Other gases are physically entrained by the batch combustion heat sources. Most of the gas escapes during the initial phase of melting, but some becomes entrapped in the melt. A primary objective of refining is to provide sufficient time and temperature conditions for substantial portions of these entrapped gases to be eliminated from the melt. Because elevated temperatures expedite the rise and escape of gaseous inclusions, the highest temperatures of the melting process are typically provided in the refining zone. Additionally, thermal conditions are conventionally controlled in the refining zone to maintain recirculating flows of the molten glass in order to provide adequate residence time and to assure that the throughput stream passes through the region at high temperatures, where gases are released into the space above the melt, and that unrefined portions of the melt are directed away from the throughput stream. Additionally, the refining stage may be employed to assure dissolution of any remaining solid particles of the batch. Furthermore, the recirculation established during refining can be useful in homogenizing the melt. It would be desirable to optimize the achievement of at least some, and preferably all, of these objectives of refining when coupled to a discrete liquefying stage as in U.S. Pat. No. 4,381,934. Prior to this invention, it was found that feeding liquefied material to a recirculatory refining tank has a tendency to create short-circuit flow patterns, whereby incoming material passes relatively quickly into the outgoing product stream, thus providing inadequate residence time for refining.
A difficulty arises from the fact that material discharged from a liquefying stage is only partially melted, typically being in a substantially foamy condition with unmelted solid particles. When such material is passed to a pool of molten glass in a refining furnace, the material tends to stratify near the surface of the pool. This stratified material has been found to not respond to the recirculating flows within the main portion of the pool that assure adequate residence time and temperature exposure to accomplish the refining step. Accordingly, discharging material from a liquefying stage directly to a recirculating refining furnace as shown in U.S. Pat. No. 4,381,934 has been found to yield inadequate refining.
Another problem is that maintaining the desired convection flow patterns in the refiner is more difficult when the material entering the refiner is liquefied. This is because in a conventional tank type melting and refining furnace the unmelted batch materials fed onto the molten pool serve as a heat sink at one end of the pool, thereb creating a downward flow in that region which contributes to sustaining a strong circulation pattern. Such an effect is not present to as great an extent when the batch materials are liquefied at a separate location. When there is insufficient recirculation in the refiner, the probability increases that a portion of the material will pass quickly to the outgoing product stream, thereby contaminating the product with inadequately refined glass.
In prior art glassmaking furnaces, the melt usually progresses from a relatively large melting chamber into smaller or narrower vessels for refining and conditioning, often passing through a constricted passageway in going from one chamber to the next. The following U.S. patents show typical compartmentalized glassmaking furnaces: U.S. Pat. Nos. 1,941,778; 704,040; 2,254,079; 2,808,446; 3,399,047; 3,897,234; 4,099,951; and 4,195,982. Heating molten glass in narrow passageways leading to refining chambers may be seen in U.S. Pat. Nos. 2,926,208; 2,990,438; 3,499,743; 4,011,070; 3,261,677; 3,415,636, and 2,691,689. None of these patents recognizes the effects on the efficiency of the refining process that have been found to be attributable to the thermal condition and physical orientation of the stream entering the refiner.